The invention relates to a contact pin for a printed circuit board, in particular a contact pin comprising a compliant portion for mounting the contact pin in a hole in the printed circuit board.
U.S. Pat. No. 3,634,819 discloses a contact pin with a compliant portion having two legs extending in the longitudinal direction and joining a solid contact pin portion at both ends. The legs are displaced outwardly in opposite directions so that the legs are separated by a slot along their whole length. During insertion the legs move towards each other through the slot. Due to the slot the cross section of the legs is decreased and thereby the strength of the legs is also decreased.
U.S. Pat. No. 4,186,982 discloses a contact pin with a compliant portion having two legs extending in the longitudinal direction of the contact pin and joining a solid contact pin portion at both ends. The legs are displaced outwardly in opposite directions wherein the opposite surfaces of the legs are partially overlapping each other along their whole length. During insertion the opposite surfaces of the legs slide along each other along their whole length thereby providing a sliding friction force to be overcome during insertion resulting in a high insertion force. This high insertion force may cause a cold weld between the coating of the hole wall and the coating of the compliant portion of the contact pin. Such a cold weld is disadvantageous because replacement of the contact pin could easily cause an unrepairable damage to the wall of the hole. Moreover, as the diameter of the hole will vary during the lifetime of the printed circuit board due to temperature effects for example, this sliding friction force adversely effects the long term retention force of the contact pin.
Furthermore, due to the frictional engagement of both legs or spring members, the latter do not work independently from each other resulting in an intermittently movement causing a random spring rate. The asymmetrical configuration of the compliant section is responsible for a torsion of the pin ends after insertion, causing damage to the mating socket contact. Although an anti-torsional feature can be built in, as described in U.S. Pat. No. 4,186,982, it is very hard to incorporate this in a miniaturized version (for small holes) without loosing the elastic properties of both legs.
EP-A-0 141 492 discloses a contact pin with a compliant portion having two legs arranged in the same manner as in U.S. Pat. No. 3,634,819. Further a center leg is provided in the slot between the two outer legs which center leg is displaced radially outwardly in order to prevent torsion during insertion. As in the contact pin of U.S. Pat. No. 3,634,819, the strength of the outer legs is decreased by the slot between these legs. The center leg does not attribute to the retention force in a significant manner.
The increasing integration of functions in integrated circuit devices leads to a demand for a higher number of connections on a printed circuit board. Further it is desired to restrict the area available for the contact pins so that more place will be available for the conductor tracks. Therefore it is desirable to use holes with a smaller diameter, for example approximately half of the size of the currently used holes. However, especially for small holes in a printed circuit board, the flexibility of the compliant portion of the contact pin has to be high in order to prevent damages to the plating of the hole. Furthermore sufficient spring strength has to be foreseen in order to provide a sufficient retention force.
With respect to a possible application of the prior art, U.S. Pat. No. 3,634,819 and EP-A-0 141 492, for small holes (0.5 mm) the cross-sectional area of the legs has to be further decreased thereby limiting the springabilities and strength of both legs to such an extent that, insufficient retention force is obtained, whereby an electrical connection is not guaranteed anymore during the lifetime of the system. In view of the small dimensions of the contact pin suitable for such small holes, the force required to overcome any possible friction forces, as appearing in U.S. Pat. No. 4,186,982, could cause overstressing of the legs of the contact pin and damages to the hole. It will be clear that overstressing of the legs results in a low retention force.